VATICAN CITY  Pope Francis laid out the priorities of his pontificate during his installation Mass on Tuesday, urging the princes, presidents, sheiks and thousands of ordinary people attending to protect the environment, the weakest and the poorest and to let tenderness "open up a horizon of hope."

A flag from Argentina waves in the foreground as Pope Francis, bottom, is driven through the crowd prior to his inaugural Mass, in St. Peter's Square at the Vatican, Tuesday, March 19, 2013. Pope Francis thrilled tens of thousands of ...
— / AP

Pope Francis blesses a man prior to his inaugural Mass, in St. Peter's Square at the Vatican, Tuesday, March 19, 2013. Pope Francis thrilled tens of thousands of people on Tuesday gathered for his installation Mass, taking a long round-about ...
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Faithful gather in St. Peter's Square at the Vatican, Tuesday, March 19, 2013. Pope Francis has urged princes, presidents, sheikhs and thousands of ordinary people gathered for his installation Mass to protect God's creation, the weakest and the poorest of ...
— / AP

It was a message Francis has hinted at in his first week as pontiff, when his gestures of simplicity often spoke louder than his words. But on a day when he had the world's economic, political and religious leadership sitting before him on the steps of St. Peter's Basilica for the official start of his papacy, Francis made his point clear.

"Please," he told them. "Let us be protectors of creation, protectors of God's plan inscribed in nature, protectors of one another and of the environment."

The Argentine native is the first pope from Latin America and the first named for the 13th-century friar St. Francis of Assisi, whose life's work was to care for nature, the poor and most disadvantaged.

The Vatican said between 150,000-200,000 people attended the Mass, held under bright blue skies after days of chilly rain and featuring flag-waving fans from around the world.

In Buenos Aires, thousands of people packed the central Plaza di Mayo square to watch the ceremony on giant TV screens, erupting in joy when Francis called them from Rome, his words broadcast over loudspeakers.

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"I want to ask a favor," Francis told them. "I want to ask you to walk together, and take care of one another. ... And don't forget that this bishop who is far away loves you very much. Pray for me."

Back in Rome, Francis was interrupted by applause several times during his homily, including when he urged the faithful not to allow "omens of destruction," hatred, envy and pride to "defile our lives."

Francis said the role of the leader of the world's 1.2 billion Catholics is to open his arms and protect all of humanity, but "especially the poorest, the weakest, the least important, those whom Matthew lists in the final judgment on love: the hungry, the thirsty, the stranger, the naked, the sick and those in prison."

"Today amid so much darkness we need to see the light of hope and to be men and women who bring hope to others," he said. "To protect creation, to protect every man and every woman, to look upon them with tenderness and love, is to open up a horizon of hope, it is to let a shaft of light break through the heavy clouds."

After the celebrations die down, Francis has his work cut out for him as he confronts a church in crisis: Retired Pope Benedict XVI spent eight years trying to reverse the decline of Christianity in Europe, without much success.

While growing in Africa and Asia, the Catholic Church has been stained in Europe, Australia and the Americas by sexual abuse scandals. Closer to home, Francis is facing serious management shortcomings in a Vatican bureaucracy in dire need of reform.